The present disclosure relates generally to exhaust gas sensors capable of detecting and measuring exhaust gas compositions, and more particularly, relates to an improved compact design for exhaust gas sensors.
Automotive vehicles with an internal combustion engine have an exhaust system that includes a pathway for exhaust gas to move away from the engine. The temperature of the exhaust gases ranges from ambient temperature, when the engine has not been run recently, to higher than 1000xc2x0 C. Frequently used in these exhaust systems is an Exhaust Gas Oxygen (EGO) sensor assembly, which allows for a determination of a rich or lean air/fuel ratio.
The sensing element of an EGO sensor consists of a dense oxygen-conducting zirconia (ZrO2) ceramic, most commonly cylindrically shaped having an opening at one end and having a rounded closure at the other end, with porous platinum electrodes, one on the outside and the other on the inside surfaces of the cylinder. The outside electrode is covered with a porous layer of spinel or magnesia alumina oxide, typically applied either by thermal spray deposit or a co-fired slurry dip coating. The materials are commercially available from many sources. This sensing element is mounted within a housing structure that seals the inside of the cylinder from the outside of the cylinder. When the EGO sensor is mounted onto the exhaust manifold of an engine, the outer electrode is exposed to the exhaust stream whereas the inner electrode is exposed to the ambient air as a reference oxygen atmosphere. When the air/fuel ratio is lean, the EGO sensor voltage output has a small value (e.g. 50 mV) because the oxygen partial pressure in the exhaust gas is not significantly different from the oxygen pressure in the air. When the air/fuel is rich, the EGO voltage output is large (e.g., 700-900 mV) because the thermodynamic equilibrium oxygen partial pressure of the exhaust gas is many orders of magnitude smaller than that of the air reference. Consequently, when the air/fuel ratio varies from the optimal stoichiometric ratio (e.g., 14.7:1), the EGO sensor output changes abruptly between a large and a small value. This sensor output signal is conveyed by means of an associated set of electrical output leads. This signal is then used by the engine control system to adjust the air/fuel ratio being supplied to the combustion chambers of the engine to a desired air/fuel ratio, generally very close to the stoichiometric air/fuel ratio.
Most current EGO sensors also include a heater that is inserted in the air reference. The heater assists the zirconia sensor, a heated exhaust gas oxygen (HEGO) sensor, in making more precise measurements over a wide range of exhaust gas temperatures, especially when the exhaust gas temperature is low. The addition of the heater also helps to decrease the light-off time of the sensor, that is the time that it takes for the sensor to reach the minimum temperature for proper operation.
EGO sensors are typically in direct contact with extremely hot exhaust gases and exhaust gas piping, and in some designs, with supplemental heat generated by a heater rod positioned within the lower region of the sensor itself. Consequently, these sensors are designed to protect heat sensitive components of the sensor from the extreme conditions of the operating environment. Typically, the sensor components having the lowest heat resistance are located in the upper region of the sensor. Such components include the grommet or cable seal, which generally comprises an elastomeric material such as Viton(copyright) rubber, and the cable insulation, which often comprises Teflon(copyright) (i.e., a fluorocarbon polymer) or similar material. The design parameters, e.g., the size and geometry, of typical EGO sensors are similarly limited by the various temperature constraints. For example, the sensor often has a sufficient height to remove the heat sensitive components in the upper region from the hot exhaust gas.
FIG. 1 shows a conventional heated exhaust sensor 10 having three wires extending therefrom, wherein two of the wires are for heating a sensing element 36 and the third wire is an engine management system (EMS) input generating its own low voltage signal. It will be understood that a single wire exhaust sensor is grounded to the exhaust manifold. Exhaust sensor 10 comprises a three-piece housing structure comprising an upper tubular shell 19, a middle shell 26 and a lower tubular shell 31. The metal housing has a longitudinal bore 36 with a sensing element 30 disposed therein. An electrically insulating ceramic material 24 is concentrically disposed around longitudinal bore 36 to support the sensing element 30. Sensing element 30 is an exhaust sensing element of a known type with any conventional geometry, such as a generally flat elongated rectangular shape.
Lower tubular shell 31 has disposed therein a first section 30a of sensing element 30. At section 30a thereof, sensing element 30 includes an exhaust constituent-responsive structure fabricated into sensing element 30 in a known manner, preferably along with a heater rod 29 of a known type. The lower tubular shell 31 includes perforations 33 formed therein through which exhaust gas enters and contacts the sensing element 30. Exhaust gas temperatures contacting lower tubular shell may reach levels of about 1000xc2x0 C. The middle shell 26 includes wrench flats 34 and a threaded portion 35 for threading into a manifold boss of an exhaust system (e.g., pipe of manifold). Upper tubular shell 19 extends from middle shell 26 to cable seal 13. Upper tubular shell 19 houses a second section 30b of sensing element 30, connector plug 32, electrical wires 12, and cable seal 13. Upper tubular shell 19 is concentrically disposed around cable seal 13, typically an elastomeric component, and is in direct contact therewith, securing it in place. Electrical wires 12 pass through cable seal 13 into connector plug 32 to form an electrical connection with sensing element 30 through electrical terminals 16.
An outer shield 14 is concentrically disposed around upper tubular shell 19 to protect exhaust gas sensor 10 from the high temperature exhaust gas environment. Typically, outer shield 14 is concentrically disposed around upper tubular shell 19 from about the upper one-half to about the upper one-third of upper tubular shell 19. An insulating material 15, such as a breathable Teflon(copyright) (i.e., a fluorocarbon polymer) material, is disposed between outer shield 14 and upper tubular shell 19 at nearly all points. Outer shield 14 and upper tubular shell 19 are in direct physical contact at the lower end 37 of outer shield 14.
In the typical manner of use, lower tubular shell 31 of the exhaust gas sensor 10, is contacted with very hot exhaust gases generated by an engine. Contact with exhaust gases allows sensing element 30 to measure the component gases. During this process, substantial heat is undesirably conducted from lower tubular shell 31 to middle shell 26, and further to upper tubular shell 19. As upper tubular shell 19 is in direct contact with cable seal 16, substantial heat is also conducted from upper tubular shell 19 to cable seal 16 and to electrical wires 12 disposed therein. Such conduction of extreme heat from lower tubular shell 31 to cable seal 16 and electrical wires 12 is highly undesirable due to the low heat resistance of cable seal 16 and wires 12 relative to other components of exhaust gas sensor 10. As cable seal 16 and wires 12 are typically the first components of exhaust gas sensor 10 to deteriorate under high temperature operation, the reduction of heat conduction to them is advantageous. The reduction of heat conduction to the wires is typically gained by increasing the distance the wires are from the lower tubular shell 31 by increasing the length of an EGO sensor. An EGO sensor with a reduced size, however, would be advantageous for use in each exhaust cylinder port for monitoring the air/fuel ratio in each cylinder.
Because of the cost and size of a typical EGO sensor, one sensor is usually installed after an exhaust manifold converges to one pipe (xe2x80x9cengine outxe2x80x9d ) in a vehicle exhaust system to monitor the air/fuel ratio of the engine as a whole, rather than in any one of the runners which would sense exhaust gas in any cylinder individually. Another sensor may be installed after the catalytic converter (xe2x80x9cpost converterxe2x80x9d ). The cost and dimensions of current EGO sensors do not facilitate the installation of an EGO sensor in each cylinder port of an exhaust manifold where monitoring the air/fuel ratio of each cylinder individually is advantageous.
What is needed in the art is an improved EGO sensor design that is small enough, cost effective, durable and serviceable for use in each exhaust manifold port for each cylinder of an engine.
EGO sensors are desirably employed in connection with each cylinder of an engine enabling an engine management system (EMS) to optimize the air/fuel ratio in each cylinder and thereby improve performance and fuel economy of each of the cylinders individually and therefore the engine as a whole. An EGO sensor for a multiport application comprises: a sensing element disposed within a housing having a lower shell portion, a middle shell portion, and an upper shell portion, wherein said sensing element extends from said lower shell portion through at least a portion of said middle shell portion; a connector plug disposed within said upper shell portion; one or more electrical wires extending from said connector plug and through an opening in said upper shell portion disposed on a side of said connector plug opposite said middle shell portion, and said opening forming a seal in said upper portion disposed around said one or more electrical wires.